1. Field of the Invention
The present invention relates to a zoom lens system, and in particular, relates to a high-magnification zoom lens system which is suitable for an interchangeable lens system of an SLR camera, and has an zoom ratio of about 10.
2. Description of the Prior Art
A zoom lens system having a zoom ratio of about 10 proposed in, for example, Japanese Unexamined Patent Publication No. 1-191819, includes a positive first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group, in this order from the object. In this high-magnification zoom lens system, the negative second lens group and the positive fifth lens group remain stationary upon zooming, and the overall length of the zoom lens system is long. Furthermore, the angle of view 2xcfx89 at the short focal length extremity is small to the extent of about 62xc2x0.
Japanese Unexamined Patent Publication No. 10-133109 also proposes a zoom lens system which also includes a positive first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group, in this order from the object. In this five-lens-group zoom lens system, all of the lens groups move upon zooming, and the traveling distance of the first lens group is long, which is disadvantageous for miniaturization of the zoom lens system.
The present invention provides a high-magnification zoom lens system which has a short overall length, achieves a zoom ratio of about 10, and has an angle of view 2xcfx89 of about 76xc2x0.
As an aspect of the present invention, there is provided a high-magnification zoom lens system including a positive first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group, in this order from the object. Upon zooming from the short focal length extremity to the long focal length extremity, at least the positive first lens group, the positive third lens group, the negative fourth lens group, and the positive fifth lens group move from the image side toward the object side. Furthermore, the high-magnification zoom lens system satisfies the following conditions:
0.65 less than (fw*ft)1/2/f1 less than 0.95xe2x80x83xe2x80x83(1) 
xe2x88x927.0 less than (fw*ft)1/2/f2 less than xe2x88x924.8xe2x80x83xe2x80x83(2) 
0.80 less than X4/X5 less than 0.95xe2x80x83xe2x80x83(3) 
wherein
fw designates the focal length of the entire high-magnification zoom lens system at the short focal length extremity;
ft designates the focal length of the entire high-magnification zoom lens system at the long focal length extremity;
f1 designates the focal length of the positive first lens group;
f2 designates the focal length of the negative second lens group;
X4 designates the traveling distance of the negative fourth lens group upon zooming from the short focal length extremity to the long focal length extremity; and
X5 designates the traveling distance of the positive fifth lens group upon zooming from the short focal length extremity to the long focal length extremity.
The high-magnification zoom lens system preferably satisfies the following condition:
0 less than (L(1xe2x88x923)W+X3xe2x88x92X1)/fw less than 0.3xe2x80x83xe2x80x83(4) 
wherein
L(1xe2x88x923)W designates the distance from the most image-side surface of the positive first lens group to the most object-side surface of the positive third lens group, at the short focal length extremity;
X3 designates the traveling distance of the positive third lens group upon zooming from the short focal length extremity to the long focal length extremity; and
X1 designates the traveling distance of the positive first lens group upon zooming from the short focal length extremity to the long focal length extremity.
The high-magnification zoom lens system performs focusing from an object at infinity to a close-distance object by moving the negative second lens group toward the object, and satisfies the following conditions:
0.75 less than |m2t| less than 1.0xe2x80x83xe2x80x83(5) 
0.05 less than D1W/fw less than 0.10xe2x80x83xe2x80x83(6) 
wherein
m2t designates the lateral magnification of the negative second lens group at the long focal length extremity;
fw designates the focal length of the entire high-magnification zoom lens system at the short focal length extremity; and
D1W designates the air distance between the positive first lens group and the negative second lens group at the short focal length extremity.
In order to correct aberrations, the high-magnification zoom lens system preferably includes a lens element on which an aspherical surface is formed in at least two of the negative second lens group, the positive third lens group and the positive fifth lens group. Providing aspherical surfaces in these lens groups enables the correcting of aberrations in an effective manner, and can reduced the number of lens elements.
The high-magnification zoom lens system preferably satisfies the following conditions:
1.0 less than f3*N3xe2x88x921/R3xe2x88x921 less than 2.0xe2x80x83xe2x80x83(7) 
62 less than xcexd3xe2x88x921xe2x80x83xe2x80x83(8) 
wherein
f3 designates the focal length of the positive third lens group;
N3xe2x88x921 designates the refractive index, with respect to the d-line, of the most object-side lens element of the positive third lens group;
R3xe2x88x921 designates the radius of curvature of the most object-side lens element of the positive third lens group; and
xcexd3xe2x88x921 designates the Abbe number of the most object-side lens element of the positive third lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-259448 (filed on Aug. 29, 2001) which is expressly incorporated herein in its entirety.